DNA helicases unwind duplex DNA molecules and are involved in DNA replication, recombination and repair. The RuvB protein is induced in E. coli as part of the SOS response to DNA damage. All sorts of electron microscopy and image analysis have been used to show that in the presence of ATP it forms a dodecamer on double-stranded DNA. Two stacked hexameric rings encircle the DNA and are oriented in opposite directions. This structural arrangement may be common to other helicases. (Stasiak, Tsaneva, West, Benson, Yu and Egelman. 1994. PNAS 91, 7618-7622.)